Data storage devices employing rotating magnetic or optical media disks are known for high capacity, low cost storage of digital data. Such disks typically contain a multiplicity of concentric data track locations, each capable of storing useful information. The information stored in each track is accessed by a transducer head assembly which is moved among the concentric tracks. Such an access process is bifurcated into two operations. First, a track seek operation is accomplished to determine the track that contains the data to be recovered and, second, a track following operation is accomplish to maintain the transducer in precise alignment with the track as the data is read therefrom. Both these operations are also accomplished when data is to be written by the transducer head assembly to a specific track on the disk.
Physical positioning of the transducer head assembly is typically accomplished by a rotary actuator assembly which supports the transducer assembly at one end of the rotary actuator assembly. At an opposing end of the actuator assembly is an actuator motor that causes the actuator assembly to pivot about a centrally located axis and position the transducer head assembly over the disk. Control circuitry controls the actuator motor such that the head assembly is accurately positioned amongst the concentric tracks on the disk. Typically, the actuator motor forms a portion of a continuously positionable system (servo system) that uses a closed loop servo circuit to control the position of the transducer assembly relative to the tracks on the disk, i.e., the servo system continuously adjusts the position of the actuator assembly based upon servo information read by the transducer assembly from the disk.
In high-capacity disk drives such as those disclosed in U.S. Pat. Nos. 5,235,478 and 5,073,833, transducer head assemblies typically contain two transducers, one for reading information from the disk and another for writing information to the disk. The read transducer is a magneto-resistive head and the write transducer is an inductive head. As is well-known in the art, a magneto-resistive head is much more sensitive to recorded magnetic flux transitions than an inductive head. As such, utilization of a magneto-resistive head enables the track density to be significantly increased over the track densities associated with disk drives that use inductive heads to both read and write data to the disk.
As for the physical arrangement of the heads, the two heads are typically linearly arranged upon a slider, one head behind the other, with a relatively small gap between the two heads. Alternatively, the centerlines of each of the two heads are laterally offset from one another by a relatively small distance. Such an offset can be utilized to minimize a radial distance that the actuator assembly must be displaced or "micro-jogged" between centerlines when switching from reading to writing, or from writing to reading operations. The slider upon which the heads are mounted forms a portion of the transducer head assembly mounted to one end of the actuator assembly.
Generally, the head arrangement described above is known as a write-wide, read-narrow head arrangement. Specifically, the inductive write head has a electrically wide linear region, e.g., approximately one-fourth of a nominal track width (track pitch). On the other hand, the magneto-resistive read head has a linear region that is approximately 16% of the track pitch. Because of this narrow reading width, disk drives, such as that disclosed in U.S. Pat. No. 4,783,705 commonly assigned to the assignee hereof, utilize servo sectors comprising multiple servo bursts arranged within a single track. In effect, the track pitch is different (smaller) for the servo sectors as compared to the data sectors. As such, the narrow effective width of the read head will always pass over at least one of the multiple servo bursts even though the head may be mistracking the center of the data track. Consequently, by reading the servo bursts beneath the read head, the servo system can reposition the head in the center of the data track.
One disadvantage of using a transducer head assembly having two, spaced apart transducer heads on a rotary actuator assembly is that, as the transducer head assembly is positioned relative to the concentric tracks, a skew angle between the transducers heads becomes apparent. Specifically, if the transducer heads were perfectly aligned, one behind the other, over a given track near the outer diameter of a disk, as the transducer head assembly is moved toward the inner diameter of the disk, the transducer heads skew relative to an underlying track. To compensate for this effect, the two heads are laterally offset from one another. As such, the two heads are typically aligned with a track near the center of the disk and become misaligned (skewed) with tracks on either side of the central track. Consequently, misalignment can be as much as +14% at the inner diameter of the disk and -12% at the outer diameter of the disk.
To further compensate for the skew angle, special circuitry is used to coordinate the read and write functions during track seek and follow operations. In particular, during a seek operation, the read head is used to read embedded servo information recorded within each track on the disk. The servo information is recorded in one or more so-called servo sectors. This servo information informs the actuator control electronics of the specific track number the transducer head assembly is presently passing over and the relative alignment of the head with that particular data track.
Once the desired track is found, the servo information read by the read head is used within a closed loop servo controlling the actuator voice coil motor in order to move the actuator structure to minimize a position error signal (PES) and thereby accurately align the read head with the center of the data track. Thereafter, the read head can read the data present in one or more data segments that follow each servo sector during data reading operations. Typically, the data segments are aligned with the so-called centerline of the track.
Rotary actuators inherently cause a skew angle to be manifested between the head structure and the concentric data tracks, because the head is positioned along an arc, rather than along a straight radial line. In addition, with separate write and read elements arranged in tandem within the data transducer head structure, a further skew angle or offset between the write element and the read element may be present at any particular radial track location. If a data writing to disk operation is to be carried out at the data segment being followed by the read element within the track, the write gap will be offset from the track centerline by an amount related to the head skew, and the actuator assembly must be moved a distance, known as the "micro-jog distance", in order to bring the write head into alignment with the track centerline.
In this manner, when a servo sector is encountered at the beginning of a write operation, the read element reads the head position information from within a servo sector, and the servo control loop determines the micro-jog distance. The transducer head assembly is then micro-jogged to place the write element into alignment with track centerline before the writing operation is carried out. During a writing operation spanning several servo sectors, when a next servo sector is encountered, the transducer head read element is offset by the micro-jog distance from the track centerline, and the servo loop must somehow follow the track centerline with an offset read element. This is a non-trivial design problem as each micro-jog interval requires a finite time to move the actuator structure. Tolerances or gaps, or additional disk revolutions must be provided in order to accommodate head repositioning during writing operations thus increasing the time spent on head repositioning and increasing the servo overhead of the storage disk. Thus, it is not practical to microjog the data transducer assembly back and forth when each servo sector interrupts a data track during an extended data writing operation spanning several data areas interrupted by servo sectors.
To ensure that the transducer head assembly maintains alignment with the data segments and can detect where within the track the data segments begin, two offset address fields are typically used. These address fields are also known in the art as identification (ID) fields. Such offset address fields are disclosed in U.S. Pat. No. 5,257,149, issued Oct. 26, 1993. This patent teaches using two address fields each of which contain a track number and position information concerning a particular track that is associated with the two fields. A read address field is aligned with the centerline of the data track. The read address field is followed by a write address field. The write address field is radially offset from the track centerline by a predefined distance. This predefined distance is equivalent to the micro-jog distance for that particular track. As such, the predefined distance varies with track position on the disk. The write address field is followed by a data field that is aligned with the track centerline. Using such a radially offset write address field ensures that, during data writing, the read head is centered over the write address field while the write head is centered over the data track.
During data read operation, the disk drive taught by the '149 patent uses the read address field to identify a particular track to read data from and thereafter, reads data from the data field following the read address field. However, during data write operation, the actuator assembly offsets the read head from the track center by the predefined micro-jog distance such that the read head is positioned over the write address field. As a result, although the read head is offset from the track center, the write head is positioned over the data field located at the center of the track. As such, the disk drive taught by the '149 patent compensates for the skew angle and any fixed head offset built into the transducer head assembly. Consequently, a micro-jog is only necessary to initially align the write head with the data track during write operations.
However, in small disk drives, such as a 2.5 inch form factor disk drive, the very small storage area of the 2.5 inch diameter data storage disk makes it desirable to use as much of the disk surface as possible to store data. As such, using two address fields requires and consumes recordable surface area of the disk that could otherwise be used to store user data, thereby decreasing the servo data overhead.
Therefore, there is a need in the art for a dual-head disk drive that uses only a single identification field for both read and write operations. Further, there is a need in the art to organize the servo information within a servo sector such that the absolute position of the transducer head assembly can be rapidly determined.